(1) Field of the Invention
The present invention generally relates to the continuous casting of molten metal and in particular to the continuous casting of molten steel. In particular, the present invention relates to tundish vessels and, more particularly, to tundish impact pads designed to inhibit or reduce turbulent flow of molten metal within the tundish.
(2) Description of Related Art
A process for the continuous casting of molten metal is well known in the art. This process will now be described with reference to steel, but it is to be understood that the present invention is not limited to the continuous casting of molten steel. In particular, the present invention can also be used with other alloys or molten metals such as iron or even non-ferrous metals. In this known process, molten steel is poured into a transport ladle that conveys the molten metal to the casting apparatus. The ladle is provided with a discharge orifice in its bottom wall. Generally a sliding gate arranged just below the discharge orifice is used to control the flow of molten steel towards a tundish. To prevent the oxidation of the molten steel discharged from the ladle into the tundish, a ladle shroud is generally connected to the sliding gate to transfer the molten steel sheltered from the surrounding atmosphere. The bottom end of the ladle shroud is normally immersed into the tundish steel bath.
The tundish is an intermediate metallurgical vessel receiving the molten steel discharged from the pouring ladle. In turn, the tundish distributes the molten steel into one or more casting molds arranged below the tundish. The tundish is used for separating slags and other contaminants from the molten steel. The molten steel flows along the tundish toward one or more outlets discharging the molten steel into the said one or more casting molds. The length of the tundish is selected to provide a time of residence of the metal in the tundish sufficient to allow separation of the inclusions as a floating slag layer. The flow of molten steel discharged from the tundish is generally controlled, most often with a stopper, and, as for the steel discharged from the ladle, is generally shrouded with a nozzle conveying the molten steel from the tundish into the casting mold.
The present invention is of particular value for a specific tundish design wherein the molten steel stream is introduced into the tundish in a pour area consisting in a side extension of the tundish main body. This side extension is in fluid connection with the tundish main body. Such a tundish is often called T-shaped tundish (when viewed in plan, the cross-bar or top of the “T” corresponds to the main body of the tundish and so is of greater length than the tail or vertical of the “T”). The area inside the tundish in the region of the tail of the “T” (the side extension) is usually the pour area where molten steel is introduced into the tundish. This region, therefore, normally has a special erosion-resistant impact pad on the floor. In a variant of the T-shaped tundish (sometime called h-shaped tundish), the tail or pour area is arranged obliquely (or even parallel) with respect to the tundish main body. In the context of the present invention, any such tundish will be designated as T-shaped tundish.
This type of tundish is generally provided with an even number of outlets which are symmetrically arranged in the bottom floor of the tundish with respect to the tundish center. For example, in the case of a bloom caster, four to six outlets are generally provided in the tundish floor.
One significant problem often encountered with this type of tundish is the difference of flow velocity of the streams discharged from the different outlets. In other words, the residence time of the molten steel in the tundish is significantly longer for the outlets further away from the tundish center than for the outlets which are closer to the tundish center. In turn, this gives rise to steel quality problems and more particularly to a significant difference of quality between the steel discharged from the different outlets.
Another problem is the speed of the transition at ladle change. Indeed, due to the different velocities of the streams discharged through the different outlets, the transition is much longer for the outer streams than for the center streams.
Pouring pads placed within tundishes have been widely used to prevent damage to the working and safety linings of a tundish by the force of the incoming stream of molten metal. The kinetic energy of the incoming stream of molten metal also creates turbulence which can spread throughout the tundish if the flow of molten metal is not properly controlled. Many times, this turbulence has a detrimental effect on the quality of cast products formed from metal taken from the tundish. More specifically, turbulent flow and high velocity flow within the tundish can, for example, have the following harmful effects:
1. excessive turbulence can disturb the steel surface and promote emulsification of the slag at ladle changes or during operation of the tundish with a relatively low level of molten metal;
2. high velocities produced by turbulent flow in the pouring area can cause erosion of the working lining of the tundish which is typically comprised of a refractory material having a much lower density than impact pads;
3. highly turbulent flow within the tundish can impede the separation of inclusions, especially inclusions less than 50 microns in size, due to the fluctuating nature of such turbulent flows;
4. high speed flows may also increase the possibility of slag being directed into a mold through increased vortexing of the molten metal in the tundish which draws slag downwardly toward the outlet;
5. turbulent flow within the tundish may result in disturbance of the slag/metal interface near the top of the metal bath and thereby promote slag entrainment as well as the possibility of opening up an “eye” or space within the slag layer which can be a source of reoxidation of the molten metal;
6. high levels of turbulence in the tundish can be carried down into the pouring stream between the tundish and the mold. This can cause “bugging” and “flaring” of the pouring stream which thereby lead to casting difficulties;
7. high velocity flow in the tundish has also been attributed to a condition known as “short circuiting”. Short circuiting refers to the short path a stream of molten metal may take from the ladle to the impact pad to the nearest outlet in the tundish. This is undesirable since it reduces the amount of time inclusions have to be dissipated within the bath. Instead, the high velocity flow sweeps relatively large inclusions down into the mold where they reduce the quality of the cast products.
A typical flat impact pad causes an incoming ladle stream to impact the top of the pad and travel quickly to the side or end walls of the tundish. When the stream reaches the side and/or end walls, it rebounds upward to the surface of the tundish where it changes direction toward the center of the tundish or, in other words, toward the incoming ladle stream. This creates undesirable inwardly directed circular flows in the tundish. The opposing flows on either side or end of the tundish travel toward the center of the tundish and carry with them slag or other impurities that have floated to the surface of the bath within the tundish. As a result, these impurities are drawn toward the incoming ladle stream and are then forced downwardly into the bath and toward the outlets of the tundish. This tends to cause more of these impurities to exit the tundish into the molds thereby decreasing the quality of the products produced within the molds. In addition, it has been observed that for T-shaped tundishes, flat impact pads cause far too short residence time of the molten steel in the tundish so that the tundish cannot fulfil properly its function.
While numerous types of tundish pads have been proposed and used in the past, none of these fully address all of the problems noted above for T-shaped tundish. Examples of prior tundish pads are disclosed in the following European patents or patent applications: EP-B1-729393, EP-B1-790873, EP-B1-847313, EP-B1-894035, EP-B1-1198315, EP-B1-1490192 and EP-A1-1397221. In particular, even though the residence time of the steel in the tundish is significantly increased, short-circuiting is observed and the steel discharged through the center outlets is significantly faster than the other steel streams.